1. Field of the Invention
The present invention relates to magnetic amorphous alloys and to a method of annealing such alloys. The present invention is also directed to amorphous magnetostrictive alloys for use in a magnetomechanical electronic article surveillance or identification. The present invention furthermore is directed to a magnetomechanical electronic article surveillance or identification system employing such marker as well as to a method for making the amorphous magnetostrictive alloy and a method for making the marker.
2. Description of the Prior Art
U.S. Pat. No. 5,820,040 teaches that transverse field annealing of amorphous iron based metals yields a large change in Young's modulus with an applied magnetic field and that this effect provides a useful means to achieve control of the vibrational frequency of an electromechanical resonator in combination with an applied magnetic field.
The possibility to control the vibrational frequency by an applied magnetic field was found to be particularly useful in European Application 0 093 281 for markers for use in electronic article surveillance. The magnetic field for this purpose is produced by a magnetized ferromagnetic strip bias magnet disposed adjacent to the magnetoelastic resonator with the strip and the resonator being contained in a marker or tag housing. The change in effective permeability of the marker at the resonant frequency provides the marker with signal identity. The signal identity can be removed by changing the resonant frequency means of changing the applied field. Thus, the marker, for example, can be activated by magnetizing the bias strip, and, correspondingly, can he deactivated by degaussing the bias magnet which removes the applied magnetic field and thus changes the resonant frequency appreciably. Such systems originally (cf European Application 0 0923 281 and PCT Application WO 90/03652) used markers made of amorphous ribbons in the “as prepared” state which also can exhibit an appreciable change in Young's modulus with an applied magnetic field due to uniaxial anisotropies associated with production-inherent mechanical stresses. A typical composition used in markers of this prior art is Fe40Ni38Mo4B18.
U.S. Pat. No. 5,459,140 discloses that the application of transverse field annealed amorphous magnetomechanical elements in electronic article surveillance systems removes a number of deficiencies associated with the markers of the prior art which use as prepared amorphous material. One reason is that the linear hysteresis loop associated with the transverse field annealing avoids the generation of harmonics which can produce undesirable alarms in other types of EAS systems (i.e. harmonic systems). Another advantage of such annealed resonators is their higher resonant amplitude. A further advantage is that the heat treatment in a magnetic field significantly improves the consistency in terms of the resonance frequency of the magnetostrictive strips.
As for example explained by Livingston J. D. 1982 “Magnetochemical Properties of Amorphous Metals”, phys. stat sol (a) vol. 70 pp 591-596 and by Herzer G. 1997 Magnetomechanical damping in amorphous ribbons with uniaxial anisotropy, Materials Science and Engineering A226-228 p.631 the resonator or properties, such as resonant frequency, the amplitude or the ring-down time are largely determined by the saturation magnetostriction and the strength of the induced anisotropy. Both quantities strongly depend on the alloy composition. The induced anisotropy additionally depends on the annealing conditions i.e. on annealing time and temperature and a tensile stress applied during annealing (cf Fujimori H. 1983 “Magnetic Anisotropy” in F. E. Luborsky (ed) Amorphous Metallic Alloys, Butterworths, London pp. 300-316 and references therein, Nielsen O. 1985 Effects of Longitudinal and Torsional Stress Annealing Magnetic Anisotropy in Amorphous Ribbon Materials, IEEE Transitions on Magnetics, vol. Mag-21, No. 5, Hilzinger H. R. 1981 Stress Induced Anisotropy in a Non-Magnetostrictive Amorphous Alloy, Proc. 4th Int. Conf. on Rapidly Quenched Metals (Sendai 1981) pp. 791). Consequently, the resonator properties depend strongly on these parameters.
Accordingly, aforementioned U.S. Pat. No. 5,469,140 teaches that a preferred material is an Fe—Co-based alloy with at least about 30 at % Co. The high Co-content according to this patent is necessary to maintain a relatively long ring-down period of the signal. German Gebrauchsmuster G 94 12 456.6 teaches that a long ring down time is achieved by choosing an alloy composition which reveals a relatively high induced magnetic anisotropy and that, therefore, such alloys are particularly suited for EAS markers. This Gebrauchsmuster teaches that this also can be achieved at lower Co-contents if starting from a Fe—Co-based alloy, up to about 50% of the cobalt is substituted by nickel. The need for a linear B-H loop with a relatively high anisotropy field of at least about 8 Oe and the benefit of allowing Ni in order to reduce the Co-content for such magnetoelastic markers was reconfirmed by the work described in U.S. Pat. No. 5,628,840 which teaches that alloys with an iron content between about 30 at % and below about 45 at % and a Co-content between about 4 at % and about 40 at % are particularly suited. U.S. Pat. No. 5,728,237 discloses further compositions with Co-content lower than 23 at % characterized by a small change of the resonant frequency and the resulting signal amplitude due to changes in the orientation of the marker in the earth's magnetic field, and which at the same time are reliably deactivatable. U.S. Pat. No. 5,841,348 discloses Fe—Co—Ni-based alloys with a Co-content of at least about 12 at % having an anisotropy field of at least about 10 Oe and an optimized ring-down behavior of the signal due to an iron content of less than about 30 at %.
The field annealing in the aforementioned examples was done across the ribbon width i.e. the magnetic field direction was oriented perpendicularly to the ribbon axis (longitudinal axis) and in the plane of the ribbon surface. This type of annealing is known, and will be referred to herein, as transverse field-annealing. The strength of the magnetic field has to be strong enough in order to saturate the ribbon ferromagnetically across the ribbon width. This can be achieved in magnetic fields of a few hundred Oe. U.S. Pat. No. 5.469,140, for example, teaches a field strength in excess of 500 Oe or 800 Oe. PCT Application WO 96/32518 discloses a field strength of about 1 kOe to 1.5 kOe. PCT Applications WO 99/02748 and WO 99/24950 disclose that application of the magnetic field perpendicularly to the ribbon plane enhances (or can enhance) the signal amplitude.
The field-annealing can be performed, for example, batch-wise either on toroidally wound cores or on pre-cut straight ribbon strips. Alternatively, as disclosed in detail in European Application EP 0 737 986 (U.S. Pat. No. 5,676,767), the annealing can be performed in a continuos mode by transporting the alloy ribbon from one reel to another reel through an oven in which a transverse saturating field is applied to the ribbon.
Typical annealing conditions disclosed in aforementioned patents are annealing temperatures from about 300° C. to 400° C.; annealing times from several seconds up to several hours. PCT Application WO 97/132358, for example, teaches annealing speeds from about 0.3 m/min up to 12 m/min for a 1.8 m long furnace.
Typical functional requirements for magneto-acoustic markers can be summarized as follows:                1. A linear B-H loop up to a minimum applied field of typically 8 Oe.        2. A small susceptibility of the resonant frequency to fr the applied bias field H in the activated state, i.e., typically |dfr/dH|<1200 Hz/Oe.        3. A sufficiently long ring-down time of the signal i.e. a high signal amplitude for a time interval of at least 1-2 ms after the exciting drive field has been switched off.        
All these requirements can be fulfilled by inducing a relatively high magnetic anisotropy in a suitable resonator alloy perpendicular to the ribbon axis. This has conventionally been thought to be achievable only when the resonator alloy contains an appreciable amount of Co, i.e. compositions of the prior art like Fe40Ni38Mo4B18, according to U.S. Pat. Nos. 5,469,140 and 5,728,237 and 5,628,840 and 5,841,348 are unsuitable for this purpose. Because of the high raw material cost of cobalt, however, it is highly desirable to reduce its content in the alloy.
Aforementioned PCT application WO 96/32518 also discloses that a tensile stress ranging from about zero to about 70 MPa can be applied during annealing. The result of this tensile stress was that the resonator amplitude and the frequency slope |dfr/dH| either slightly increased, remained unchanged or slightly decreased, i.e. there was no obvious advantage or disadvantage for the resonator properties when applying a tensile stress limited to a maximum of about 70 MPa.
It is well known, however, (cf Nielsen O. 1985 Effects of Longitudinal and Torsional Stress Annealing on the Magnetic Anisotropy in Amorphous Ribbon Materials, IEEE Transitions on Magnetics, vol. Mag-21, No. 5, Hilzinger H. R. 1981 Stress Induced Anisotropy in a Non-Magnetostrictive Amorphous Alloy, Proc. 4th Int. Conf. on Rapidly Quenched Metals (Sendai 1981) pp. 791), that a tensile stress applied during annealing induces a magnetic anisotropy. The magnitude of this anisotropy is proportional to the magnitude of the applied stress and depends on the annealing temperature, the annealing time and the alloy composition. Its orientation corresponds either to a magnetic easy ribbon axis or a magnetic hard ribbon axis (-easy magnetic plane perpendicular to the ribbon axis) and thus either decreases or increases the field induced anisotropy, respectively, depending on the alloy composition.
A co-pending application for which one of the present inventors is a co-inventor (Ser. No. 09/133,172, “Method Employing Tension Control and Lower-Cost Alloy Composition for Annealing Amorphous Alloys with Shorter Annealing Time,” Herzer et al., filed Aug. 13, 1998) discloses a method of annealing an amorphous ribbon in the simultaneous presence of a magnetic field perpendicular to the ribbon axis and a tensile stress applied parallel to the ribbon axis. It was found that for compositions with less than about 30 at % iron the applied tensile stress enhances the induced anisotropy. As a consequence, the desired resonator properties could be achieved at lower Co-contents, which in a preferred embodiment range from about 5 at % to 18 at % Co.